A recent histological study of monkeys rapidly decompressed from deep dives has revealed that, in some cases, there are fractures of the bone surrounding the semicircular canals. In some monkeys sacrificed within a few days of their dives, there are full thickness breaks across the bony canal walls. In others, sacrificed several months after a dive, these fractures have become infiltrated by the ectopic growth of new bone which also invades the optic fluid spaces. It now appears that the new bone growth is caused not only by a ripping or irritation of the endosteum, which lines the inside of the bony canals, but also, at least in some cases, by a rupturing of the very hard petrous bone itself. It is difficult to avoid the conclusion that, somehow, large forces are developed within the inner ear or within the petrous bone during decompression.